It appears that developments in barrel artillery are moving towards an ever greater use of final-phase-guided or trajectory-corrected shell types. The costs for such shells are of course substantially higher than for completely unguided shells. However, at the same time, the number of shells required for a certain effect on the target is reduced radically and, in the extreme case, to one single shell which, by means of one or more trajectory corrections during its travel towards the target, can be guaranteed to provide a direct hit on the latter.
Longer ranges of fire and higher trajectory levels for barrel artillery and, thus, an associated increase in the effect exerted on the projectiles by winds which are difficult to assess from the ground and also, to some extent, at least as regards anti-aircraft defense, faster-moving types of targets such as planes and missiles manoeuvring at supersonic speed, have also greatly increased the need to be able to correct the projectile trajectories while the shells are travelling towards the target has increased at great pace. At the same time, miniaturization of the electrotechnics and developments in the target tracking and computing components in the area of fire-control equipment made it possible to reduce the sizes when constructing projectiles whose projectile trajectories can be corrected.
The trajectory correction of both barrel projectiles and also rockets and missiles is in itself previously known. However, there are a number of different methods and apparatuses for producing these trajectory corrections. One method is to use so-called impulse motors, for example gas generators provided with outlet nozzles at suitable positions and with short or very short burning times which give the projectile, rocket or missile a trajectory correction in the form of a laterally directed push. If the outlet of the impulse motor lies in the center of gravity of the projectile, rocket or missile, the trajectory correction is obtained in the form of a lateral component of velocity, for example a velocity vector change. Barrel projectiles whose trajectory can be corrected must be stabilized with fins and provided with a slight rotation in the trajectory so that the desired direction of correction can be chosen at any given time. This in turn means that only very short burning times can be permitted for the trajectory-correcting impulse motors, since the latter may only operate while they are in correct alignment. This means that each impulse motor of this type can only be allowed to have a burning time of the order of magnitude of no more than a few milliseconds, which at the same time limits the maximum trajectory correction which can be obtained with each impulse motor, which itself means that several impulse motors may be needed to produce a single trajectory correction and, in addition, with today's technology, it is possible to effect several separate trajectory corrections while the projectile is moving towards the target. Rotating projectiles afford the advantage that a number of impulse motors can be arranged around the periphery of the projectile in such a way that all have their gas outlet in the center of gravity of the projectile.
As already mentioned, the burning time for each impulse motor in rotating projectiles must be kept extremely low, and then normally less than one millisecond. This means that the motors must be charged with a fast-burning powder, which in itself is not any problem. On the other hand, it has proven substantially more difficult to produce an instantaneous cross-ignition over a sufficiently large part of the burning area of the propellent powder charge available in the initial stage. If the ignition is effected along too small a surface, then, in the case of the extremely fast-burning powder in question here, the effect is that the rapidly increasing gas pressure expels large quantities of uncombusted powder through the outlet nozzle. Thus, the desired trajectory-correcting effect is of course not achieved. The problem of achieving rapid cross-ignition then further increases in the case of smaller projectile sizes, since, with smaller sizes, for example 40-mm AA shells, reasons of space force one to use narrow, elongate charges for the impulse motors.